Artificial heart pacemakers are widely used in the medical field for the treatment of a number of different heart disorders. Many different types of pacemakers, operating in many different modes, have been developed over the years for the treatment of specific disorders or conditions. The control of the operation of most types of modern pacemakers depends in part on the reliable sensing of electrical signals from the patient's heart indicative of spontaneous contractions. For example, in various types of demand pacemakers, the decision whether to deliver a stimulating pulse to the heart is based upon whether a spontaneous heartbeat has occurred within a predetermined time interval from the preceding beat. The spontaneous beat is detected by electrical signals which are part of the ECG, generated within the heart and transmitted over the pacemaker lead to sense amplifier circuitry within the pacemaker. Upon detection of a spontaneous depolarization, the pacemaker timing circuits are reset and the output circuits are inhibited from delivering a stimulating pulse for that heartbeat cycle. Reliable detection of spontaneous heart depolarizations is important in demand type pacemakers in order to avoid unwanted and possibly harmful effects of competitive pacing. If the timing output circuits are not reset following a spontaneous contraction, there is a danger of delivering a stimulating pulse when none is needed. Further, the stimulating pulse might occur shortly after the spontaneous depolarization, during the vulnerable period of the ventricle.
It is generally necessary to provide some type of filtering circuits for the sensing amplifier in a demand pacemaker, so that it responds only to the desired portion of the ECG signals from the heart. For example, in the case of a demand pacemaker sensing and pacing in the ventricle of the heart, it is generally necessary that the pacemaker respond only to the R-wave, or QRS complex of the electrocardiogram indicative of a ventricular depolarization, and not any other signals originating in the heart or elsewhere that might be picked up by the ventricular lead. Examples of such other signals include 60 Hz noise, muscle noise, and spurious RF signals. Similarly, in the case of a pacemaker having a demand operation associated with the atrium, or in the case of an atrial synchronous pacemaker, it is important that the atrial sensing circuits respond only to the P-wave of the ECG which is indicative of an atrial depolarization. Filtering circuits are often used for both atrial and ventricular sensing circuits, but of course the filter characteristics would be different in each case since the frequency spectrums of P-waves and R-waves are different.
The widths of the P-waves and R-waves vary from patient to patient. The width affects the band of frequencies wherein the maximum energy content resides. Thus the frequency content of P-waves and R-waves varies from patient to patient. Furthermore, it is known that the width of the P-waves and R-waves may vary in time for a given patient. This is particularly true immediately after a myocardial infarction.
In the design of atrial or ventricular sensing amplifier filtering circuits, various types of filters such as multiple pole active filters have been used. Because of the variability of the frequency band of maximum energy in different patients, it has been necessary to use filtering circuits designed to provide a bandpass filter covering the entire frequency range for all patients. The bandwidth of the filter as well as the gain of the associated amplifying circuits are selected to give the required degree of sensitivity within the selected passband. Selecting the circuit parameters involves choices and compromises, because making the passband too wide opens the circuit up for extraneous signals, while making the passband too narrow carries the risk of failing to detect the P- or R-waves of some patients. Since it is impractical to custom design filtering circuits for pacemakers for different individuals, and since it has heretofore been impractical to provide a means for tuning the filter to the needs of a given patient, particularly in the case of an implantable pacemaker, it has therefore been necessary to provide a bandpass wide enough to pass the frequency components of all patients. The result has been that the filter characteristics are less than optimum for any given patient.
It will be appreciated that in the case of conventional electronic filters it would be necessary to change a great number of component values in order to change the bandpass characteristics, and this would require so many additional components and switches as to be impractical in an implantable pacemaker. Adding additional components usually has a serious negative impact on an implantable device in terms of current drain, battery life, physical size, circuit reliability, and cost.
A compromise design is often used for atrial and ventricular sensing filters, wherein the filter characteristics are designed to pass the P-waves or R-waves, respectively, of the majority of patients to be encountered, and a gain or sensitivity adjustment is provided to take care of patients whose frequency bands fall marginally at either end of the passband. While providing a workable solution, such a compromise still suffers the disadvantages of providing less than optimum filtering characteristics and excessive gain outside the P- or R-wave band for a given patient, which has the potential of making the sensing circuit susceptible to signals other than desired P- or R-waves picked up by the corresponding pacemaker lead.
The present invention solves the above-mentioned problems existing in the prior art by providing an ECG sensing filter for a pacemaker which is conveniently tunable to match the frequency ranges of interest (P-waves in the case of atrial sensing or R-waves in the case of ventricular sensing). It is particularly useful in implantable pacemakers, whereby the tuning can be done by remote programming after implantation. Further, in the case of patients who have suffered a myocardial infarction or who have had a change in width of the P- or R-waves for any other reason, the present invention provides for remote adjustability of the sense amplifier band pass to permit sensing which is optimal in that it maximizes the desired signal energy while minimizing undesired signal energy, i.e., noise.